1. Field of the Invention.
The subject invention relates to elongate non-linear tubes having at least two longitudinal sections that are different from one another in size, shape or composition. The tubes may be used for structural support or for incorporation into a vehicular exhaust system.
2. Description of the Prior Art.
Prior art tubes have many automotive and industrial uses. For example, prior art tubes are used to transport exhaust gases produced by an internal combustion engine. Prior art tubes also are used for structural support, such as in frames of vehicles. Most tubes have a circular cross-sectional shape. However, tubes can assume any cross-sectional shape, and many prior art tubes used for structural support are rectangular in cross-section.
Tubes often must assume a non-linear alignment. In some environments, the required non-linear alignment can be achieved with short linear lengths of tubing connected by fittings. In other environments, such as vehicular exhaust systems and structural supports, the tubes must be bent into a very precisely defined non-linear shape. For example, vehicular exhaust pipes typically must follow a very circuitous path from the engine compartment of the vehicle to a location on the vehicle where exhaust gases can be safely emitted. Precisely located and dimensioned bends are required to bypass other components of a vehicle with sufficient clearance to avoid vibration related contact and heat related damage. A small bending error at one end of an exhaust system can yield a very substantial misalignment at the opposed end of the exhaust system. Precision is even more important for tubes used in structural applications. For example, bent tubes often are used for the longitudinally extending side rails of the support frames of vehicles. Engine mounts, suspension system components and body components must be anchored to the support frame at locations that are specified to very small tolerance variations.
Most precision tube bending is carried out with a programmable bender. The typical prior art bender includes a bend die, a clamp die and a pressure die. The bend die includes an arcuate surface about which the tube will be bent. The pressure die is disposed radially outwardly from the bend die and is capable of movement in a radial direction for selectively clamping the tube against the bend die. The clamp die also engages the tube and also is disposed radially outwardly from the bend die. Initially the clamp die is adjacent to the pressure die. However, the clamp die can be rotated about the axis of the bend die to bend the tube about the outer circumference of the bend die. The angular size of the bend is determined by the amount of rotation of the clamp die from its starting position.
The prior art programmable bender also includes a collet that grips one end of the tube to be bent. The collet functions to move the tube axially and rotationally pre-programmed amounts relative to the bend die. Thus, the collet ensures that each sequential bend in a tube is at the proper spacing and the proper rotational orientation relative to the preceding bend.
Each bend causes a stretching of metal on the outer circumferential surface of the bend and a compression of metal on the inner circumferential surface of the bend. To minimize the effects of stretching, many prior art programmable benders also include a pressure die boost which effectively functions to push tubing into the bend and to thereby prevent excessive stretching. Some benders include a collet boost to assist the pressure die boost by pushing the pipe into the bend. Damage during a bending operation also can be prevented by a mandrel disposed inside the tube at the location of the bend.
Very effective prior art benders are shown in U.S. Pat. No. 4,732,025 and U.S. Pat. No. 4,959,984 both of which are assigned to the assignee of the subject invention. The bender shown in U.S. Pat. No. 4,732,025 includes all of the operative components described in the preceding paragraph. Additionally, the bender includes sensors which detect whether the rotational movement of the bend die and the clamp die has the intended effect. In particular, metallurgical characteristics may vary from one tube to the next and from one location to another along each tube. Most tubes will exhibit some springback after the pressure exerted by the clamp die has been released. The amount of springback can vary significantly depending upon metallurgical characteristics of the particular pipe. Thus, even though the bender may perform precisely the same operation for two different pipes, the resulting bent pipes may not have the same bent shapes due to different springback. The bender shown in U.S. Pat. No. 4,732,025 senses the actual position of bent portions of the pipe, and compares the actual sensed position to a pre-specified position. If necessary, the bender shown in U.S. Pat. No. 4,732,025 can perform compensating bending operations to offset differential springback. Different tubes will exhibit different resistance to the bending and clamping forces exerted thereon. For example, some tubes will yield easily in response to bending forces and will generate excessive stretching in the outer wall of the tube. The apparatus shown in U.S. Pat. No. 4,959,984 will sense resistance and alter forces the pressure die boost and/or with the collet boost assist to effectively urge more or less of the tube into the bend. In this manner the apparatus shown in U.S. Pat. No. 4,959,984 is capable of highly precise bending due to the ability of the bender to react to sensed conditions for the actual pipe being bent.
Hydroforming has been used to deform short sections of prior art tubes. This process involves placing the short section of tube in a mold cavity conforming to the desired shape of the tube. The ends of the tube are then plugged, and fluid under pressure is directed into the plugged tube. The fluid causes the shape of the tube to change to conform to the shape of the mold cavity.
In addition to meeting certain dimensional tolerances, bent tube also must meet performance requirements. For example, certain regions of a structural tube may be particularly susceptible to vibration related damage, while other regions of the same tube may be susceptible to corrosion related damage. Some regions of a tube may include a specified material or coating primarily for aesthetic appearance. Other regions may require changes to the cross-sectional dimensions or shape. Specifications are likely to vary significantly along the length of a tube used for a vehicular frame. For example, the required wall thickness, the required cross-sectional shape and the required cross-sectional dimensions can vary significantly in accordance with the nature of the load being carried at a particular location on the tube. In other instances, the required surface coating of a supporting tube can vary significantly from one longitudinal location to the next.
Prior art tubes have been uniform along their length. This generally has required an over design of the tube so that the entire tube is made to meet the greatest load encountered anywhere along the length of the tube. Additionally, the cross-sectional shape, dimensions and surface coating for the entire bent tube typically have been dictated by the requirements at the most critical location. This occasionally requires compromises to be made at other locations along the tube.
In view of the above, it is an object of the subject invention to provide a non-linear tube that more nearly meets the specified design criteria for each location along the tube.
It is another object of the subject invention to provide a method of making a non-linear tube that more accurately meets the specifications for the entire tube.